1. Field of the Invention
The present invention relates to an apparatus and methods for drying solid articles and; more particularly, the present invention relates to the field of drying solid articles such as semi-conductor articles, i.e. wafers and more particularly, to a method of spray drying which includes a drying agent in combination with one or more other chemical agents.
2. Background
In the manufacture of computer components it often required that such components such as thin film components, computer peripherals, read-write heads and semiconductor wafers are free of contaminants and aqueous residues. For example, in semiconductor wafer processing, the wafers often exit various processing stages where water or aqueous residues are present on the wafer. It is undesirable to allow the aqueous solutions to flash dry so as to leave residuals and/or stains on such articles at the end of the drying process. Such residuals on semi-conductor wafers and like solid articles create contaminants that negatively impacts yields of usage in semi-conductors and like solid articles. The residue stains and/or contaminants degrade the performance of such articles and/or interfere with a subsequent processing step.
Various known processes have been developed in an attempt to dry the wafers and similar solid articles in a manner that removes the aqueous residue, and stains, and contaminants and static without damaging or adding contaminates to the wafer or similar solid articles. One such example of a known drying process incorporates external forces in a Spin Rinse Dryer technique, SRD. SRDs use centrifugal force to cause the aqueous residue and contaminant molecules to spin off; not altogether that different from a domestic dryer unit. SRDs are inherent residual particle generators below 0.3.mu.. The residual particulate matter is formed from the mechanics of the system and the vibration between the wafers and the boat carrier.
As the wafer and the boat carrier spin, it causes the build up of static electricity. As the surface of the article changes from a wetted state to a dry state, it becomes statically charged. The change in states attracts more airborne particulates and consequently more contamination. The performance of the article is thereby further degraded. The production yield of usable articles will further decrease because of Tribol charging on the article as the result of the article being statically charged.
Increasingly there is a decreasing dimension of line widths and structure geometries on wafers. Particulate contamination is critical to fine geometries in the sub-0.3.mu. area. It has been noted that SRD systems have substantial difficulty cleaning particulates below 0.3.mu.. Accordingly, such SRD systems are no longer an acceptable solution to the drying needs of the semi-conductor or similar industries that require fine geometries.
Another external force dryer is known as an air knife dryer. This dryer includes a plane of forced air upon the exit of the process of the article. The forced air drying, like the spin dry technique is effective but not for the smaller particles. For particulates of less than 0.5.mu., forced air drying will be ineffective. Similar to the SRD drying technique, the air knife leaves a trail of airborne particulates and statically charged particles, which promote the accumulation of contamination on the article to be dried.
Another category of dryer for such solid articles is the liquid solubilization, also known as a vapor drying process. Generally vapor dryers utilize an open vat of boiling alcohol or isopropyl alcohol (IPA) within a chamber. Alcohol vapor forms in the chamber above the boiling alcohol vat. The temperature of the vapor is approximately the same temperature as the alcohol in the vat. An article at ambient temperature, such as a wafer, is introduced into the chamber. Since the wafer is cooler than the vapor temperature, the alcohol vapor condenses on the wafer and creates a flushing action, which acts to remove aqueous residues and contaminants from the surface of the wafer. The condensed alcohol and residues are then collected and recirculated. Eventually, however, the temperature of the wafer begins to approach that of the alcohol vapor. When this occurs, the condensation and flushing action slows until very little aqueous residue or contaminants are removed. Thus, the efficiency of the process is evaluated by the amount of alcohol that can condense on the wafer prior to the wafer coming to vapor temperature. Additionally, if the wafer is has very little thermal mass, it is possible for the aqueous residue and contaminants to quickly evaporate (flash dry) on the wafer due to the heat in the chamber and have very little removed by any formed condensation. Thus, a limitation of this technique is the ability to condense alcohol on the wafer to produce a flushing action and yet not remove any residues or contaminants on the wafer.
The use of vapor drying processes also present several other considerations. The systems generally use large amounts of IPA, a flammable agent, a boiling sump, or vat, and coiling coils to assist in condensing the vapors on the wafer. The use of the open alcohol vat utilizes a large amount of fluid due to evaporation of the alcohol over the large surface of the vat. The separation of the aqueous residues from the alcohol is generally accomplished by distillation of the alcohol from the water through an alcohol reprocessor. These reprocessors are generally located in the same processing area and present issues of safety due to the highly flammable nature of the agents. Also, vapor emissions must be removed from the facility. Often this is accomplished using a burn box (scrubber) on the building, which burns off the emissions. The use of a burn box or scrubber also presents a high safety risk.
As noted above, the vapor drying processes present serious safety and environmental concerns. Even under the best of circumstances where there are no accidents, the costs of insurance will be quite high and add to the costs of manufacture.
Environmental and safety concerns are clearly quite high with a pot of boiling alcohol. Additionally, IPA is regulated as volatile organic compound by the EPA. VOC are considered to deplete ozone layer and create green house gas. In other words, IPA is regulated as VOC like freon and other hazardous chemicals.
Thus, despite IPA's superior cleaning properties, there are quite serious safety, environmental and financial concerns. Considering the risks make it desirable to seek other more attractive alternatives.
Another drying process is known as the Capillary Induced Removal (CIR) process. An example of this process is the Marangoni drying process. The Marangoni process removes aqueous residues using surface tension properties. In this process, an article is placed in a vat of water. A very thin layer of IPA is applied on top of the water. The difference in surface tension between the IPA and water creates a reverse meniscus on the article. When the article is slowly removed from the vat, the alcohol acts as a forcing function to drive the aqueous residue off the part. This procedure is limited in that the article must be removed from the vat very slowly so that surface tension is not broken. If the article is pulled out too quickly, the surface tension breaks and water ends up on the article and the article is contaminated. The process works best on planar geometries and requires a slow removal time. Thus, this procedure is not efficient for drying wafers and other articles with complex geometries, or non-planar geometries or where high-throughput of wafers is required.
Displacement drying is yet another process for drying wafer and other solid articles. In this process, water is displaced with volatile methyl siloxane, VMS and the article subsequently. The VMS leaves a film on the article and that film must be removed before the manufacturing process is complete or the article finished. In order to remove the residue from the displacement, a surfactant is used. In this case, VMS acts as a surfactant. Clearly having two steps is a serious disadvantage of the displacement drying process. Additionally, many VMS chemicals are flammable and present safety and environmental hazards as well. The additional step of rinsing off the film cuts down yield and adds consequently to the costs of manufacture. Additional costs include the environmental and safety costs, again adding to manufacturing costs.
What is needed is an apparatus and process which dries articles quickly and thoroughly and adds to the production yield over presently known methods, rather than decrease production yields. Additionally, such apparatus and method must pose the smallest possible risk to the environment and to the safety of all involved in the manufacturing process. What is also a needed such an apparatus and process that eliminates even ultra small particulates from the surface of the article without generating charged particles.